Two groups of patients referred for suspicion of acute appendicitis were compared to evaluate the accuracy of preoperative ultrasonography (US) and surgical decision-making. In one retrospective study, US was performed by trainees using a 3.5 MHz probe (219 patients). In the second prospective study, US was performed by a resident radiologist using a 5 MHz probe (144 patients). US accuracy rose from 65% to 90%, especially due to an improved negative predictive value (from 52% to 92%). The positive predictive value of US was 89%. The sonographically adjusted clinical decision to operate was correct in 85%. Thus, all patients with positive US should be operated. In contrast, clinical judgment must prevail in case of negative US findings in order to prevent surgical delay in about 11%. The negative laparotomy rate decreased by 5% only. This is probably due to the limited influence of negative US findings on surgical decision. US is to be recommended in all patients suspected for acute appendicitis when performed with an appropriate probe by an experienced ultrasonographist. However, it may not, on it self, reduce the rate of unnecessary operative procedures.